This modular, radio link was developed between 1980 and 1984 in Philips Radio Communications Ltd in Cambridge. It was an analogue link that could carry 8, 12/24, 60/72 and 120/132 speech circuits. It was typically supplied with a base-band multiplexer to break out the speech circuits.
The L700 was made in a rack-mounted and slimline equipment practice (SLEP) formats. The rack mount version is shown below.
The L700’s principal role was to carry mobile service circuits from controllers to distant fixed stations. The higher capacity variants were used to carry inter-exchange telephony. Many hill-top mounted L700s would be installed in complex region-wide mobile radio systems for emergency services or fuel and power use.
The L700 was available to operate in four frequency bands:
1790-1810 and 2290-2310MHz
The 1800/2300MHz band was a split pair allocated to the UK’s emergency services.
I designed all the RFs (with the exception of the 390-470MHz transmitter used as the TX core) and the lower-frequency interdigital filters (which were re-used from the L300 predecessor).
Here are various images of my lab designs. I designed one version of each per band over a period of about two years from 1981-1983.
Image Filter, Rat-race Mixer and Thick Film IF Amps
Complete Down Converters
Interdigital RX and TX Filters
The 390-470MHz transmitter signal was multiplied to the required frequency with a varactor.
The same design was used to obtain a local oscillator signal for the down-converter.
- The use of Teflon as a printed circuit board substrate was in its infancy.
- Through-hole plating in Teflon was particularly difficult.
- Computer-aided S-parameter RF design using the Touchstone program was in its infancy.
- Foundations for Microstrip Circuit Design by TC Edwards was my bible.
- The Teflon boards were stuck down with 3M (non-conductive) double sided tape!
Production and Installation
The 390-470MHz version of the L700 went into production in the early 80s. The other bands followed in 1983.
The L700 continued in production until the early 1990s. Pressure on the radio spectrum for mobile services meant that in the late 90s such links were allocated to higher bands such as 7GHz, 13GHz and 22GHz. The spectrum used for links by the emergency services and fuel and power was re-farmed to make way for 3G mobile networks.
I’ve tried to get hold of old production equipment. So if you have anything… Even a handbook with the circuits would be useful. Do get in touch.